Process for preparation of lead dioxide



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PROCESS FOR PREPARATION OF LEAD DIOXIDE Edgar H. Herrmann, Brooklyn, N. Y., assignor to National Lead Company, New York, N. Y., a corporation of New Jersey No Drawing. Application April 26, 1955 Serial No. 504,092 4 Claims. (Cl. 23-146) This invention relates to a new and improved method for the preparation of lead oxides, in particular lead dioxide.

Several methods for the production of lead dioxide are known to the industry. Among these may be mentioned the chlorine process, wherein the caustic slurry of red lead is treated with chlorine gas and the product Washed and dried, the nitric acid process, which involves the treatment of high grade red lead with concentrated nitric acid, followed by washing and drying, and the electrolytic process using sulfuric acid as the electrolyte with ammonium sulfate to prohibit excessive formation of lead sulfate.

As can be judged from the above brief descriptions of the prior art processes, they all suliered from certain disadvantages, among which may be mentioned the use of reagents which are hazardous and inconvenient'to handle, and the necessity of carrying out the various reactions in relatively expensive equipment. Moreover, the yields were not as high as might be desired, because of the formation of substantial quantities of by-products such as lead chloride, lead nitrate or lead sulfate.

An object of this invention, therefore, is to provide an improved method for the preparation oflead dioxide. Another object is to provide a method for production of lead dioxide which may be carried out in simple, inexpensive equipment. A further object is to provide such a method which does not involve the use of hazardous reagents or ones inconvenient to handle. Still another object is to provide a method which results in improved yields and more economical use of raw materials than was obtainable using the processes of the prior art.

Broadly, this invention contemplates a process for the production of lead dioxide which comprises the steps of reacting molten lead with caustic alkali and oxidizing to produce a fused mass of alkali plumbate dissolved in caustic alkali, granulating said alkali plumbate in the presence of limited quantities of water to produce a slurry of solid alkali plumbate in concentrated aqueous caustic alkali, separating said solid alkali plumbate from said concentrated aqueous caustic alkali, hydrolyzing said solid alkali plumbate with additional Water to produce lead dioxide and regenerate additional caustic alkali, and separating said regenerated caustic alkali from said lead dioxide. In a particularly desirable embodiment, this invention contemplates a process as just described, wherein the initial reaction between molten lead and caustic alkali is carried out in the presence of alkali nitrate, which serves to assist the reaction. When this embodiment of the invention is employed, the caustic alkali solution will contain alkali nitrite which may be recovered by evaporation and crystallization of the latter, leaving a concentrated caustic solution, which may then be dehydrated and recycled. The alkali nitrites are valuable and marketable by-products, and thus further improve the economy of the process.

While this invention applies to the alkalis in general, it will generally be most economically practical when applied to caustic soda, and for convenience, will be described hereinafter with particular reference to caustic soda.

The reaction just described, using sodium nitrate as an oxidant, and possibly also, to some extent, as an oxidation catalyst, proceeds more readily at temperatures in ice the lower range, e. g. about 400-500 C., than does the reaction carried out in the absence of sodium nitrate. Consequently, it tends to impose less severe demands on equipment, and involves a somewhat smaller fuel cost than the reaction without sodiumnitrate, and for these reasons, will generally be preferred.

However, the process as carried out using caustic soda and metallic lead alone also has its particular advantages, which may render it more attractive in some cases. Specifically, when the nitrate is omitted, the necessity of separating sodium nitrite from the caustic soda before recycling the latter is eliminated. It therefore becomes possible, at the end of the reaction between lead and canstic soda, to allow the reaction product to settle, and pour off the molten excess caustic soda in a form which is ready for reaction with additional metallic lead, no intermediate reclaiming or purification steps being necessary.

The ratio of caustic soda to lead may be varied considerably. In general, when sodium nitrate is employed, it is preferred to employ a 4- to 5-fold excess of NaOI-I over the theoretical amount required to react with the lead to form Na PbO The amount of sodium nitrate employed is also freely variable, but it is preferred to use from 0.8 to 1.0 part of NaNO, for each part by weight of Pb.

When sodium nitrate is not employed, it is necessary to depend on air oxidation to convert the lead to the plumbate, and it then becomes especially important to agitate vigorously during the reaction. Moreover, to the same end, it is desirable to increase the fluidity of the mass by employing a somewhat larger excess of NaOH over theoretical-for example an 8- to 10-fold excess. Since the caustic values are readily recoverable in either case, the use of these excesses in the reaction charge does not represent any serious economic disadvantage.

The temperature employed for the initial reaction between lead and caustic soda must, of course, be at least equal to the melting point of lead, that is about 327 C. Preferably, however, the reaction is carried out at a somewhat higher temperature, for example about 400-500 C. Substantially any temperature may be employed, providing it is not so high as to result in excessive volatilization of reagents or decomposition of products formed. Temperatures up to about 750-800 C., for example, may be employed with good success. When sodium nitrate is not employed in the reaction mixture, it is preferable to employ a somewhat higher temperature than otherwise, for example about SOD-600 C., as the reaction tends to be unduly slow below this temperature unless sodium nitrate is present.

Upon completion of the initial reaction, the resulting caustic mass is preferably granulated by quenching it in hot water, for example about C. Wat-er at this temperature and in the amounts employed (preferably from about 1.0 to 1.3 parts for each part by weight of the caustic mass) does not appreciably hydrolyze the sodium plumbate but leaves it as a hydrated solid in slurry form. Any hydrolysis that does take place is rapidly reversible in this concentrated solution. The excess caustic soda and the sodium nitrite, if present, dissolve in the water and are removed by decantation. The hydrated sodium plumbate is then further treated by the addition of more water, preferablyin amount from about 3 to 4 parts for each part by weight of sodium plumbate on a basis of dry Na lbO and raising the temperature to 95 C. This operation hydrolyzes the sodium plumbate to lead dioxide and caustic soda. This soda is in rather dilute form and may be recovered, but is generally more economically disposed of as waste alkali.

Lead dioxide prepared as above will be contaminated with minor quantities of regenerated caustic soda. Ifdesired, this excess alkali maybe removed by simply washing by decantation. When substantially all of the alkali content is removed, the lead dioxide will tend to deflocCulate and be dispersed throughout the decantation liquor. When "this occurs, the slurry may be'refiooculated by neutralizing to approximately pH=7, whereupon the system will be refiocculated and may be further washed to any extent desired. The resulting purified lead dioxide slurry may be dried on a drum drier or filtered and dried in trays or on a belt.

To illustrate the preferred method for carrying out'the process of this invention, the following examples are presented:

Example I A quantity of caustic soda amounting to 75 pounds was introducedvinto a one-half ton gas-fired kettle and heated to 450 C. For the first hour, one pound, 13 ounces of lead, and one pound, 8 ounces of sodium nitrate were added every fifteen minutes and thereafter, the same quantities of reagents were added every ten minutes until a total of 42 pounds of lead and 34 pounds, 6 ounces of sodium nitrate had been added. The reaction mixture was vigorously agitated throughout the operation. The temperature of the mass was maintained between 440 C. and 465 C. and the total additiontime was hours. Agitation was continued for an additional hour after the addition of reagents Wascomplete. The molten reaction mixture was then drawn off-and granulated in a tank containing 19 gallons of water at 80 C. with good agitation. A white granular precipitate of hydrated sodium plumbate appeared almost immediately. Agitation was continued, maintaining the tem perature at about 80 C., for 1 hour after introduction of the molten reaction mass was complete, in order to insure complete dissolution of the excess caustic soda and sodium nitrite. The slurry was then allowed to settle and the supernatant liquid was drawn ofi for recovery of sodium nitrite and caustic soda. The heavy slurry of sodium plumbate remaining in the quenching tank was then introduced into 75 gallons of water maintained at approximately 90 C. in another tank and agitated at 90 C. for approximately 2 hours to insure complete hydrolysis. The mixture was then allowed to settle and the supernatant liquid was drawn oil to waste. The heavy slurry of lead dioxide remaining in the tank wasfiltered and washed by decantation to remove excess regenerated caustic soda. On thethird washing, the slurry began to deflocculate. whereupon it was neutralized with aproximately %-'POUI1d of sulfuric acid to a pH of about 7 and decanted once more to remove the remaining sodium salts. The remaining washed precipitate was then'filtered and dried. The product was substantially pure lead dioxide, in good yield.

Example 11' One hundred twenty-five pounds of caustic soda were introduced into a kettle and heated as described in Example I to a temperature of 550 C. A total of 42 pounds of lead was added in the manner described in Example I, again with vigorous agitation, and continuing the agitation for one hour after the lead addition was complete, in order to insure complete reaction. The molten reaction mixture was then allowed to settle for 1 hour. Approximately 70% (87.5 pounds) of the total caustic employed was then poured oh and was in condition for recycling with a fresh quantity of molten lead, no further steps being necessary to purify or concentrate the reclaimed caustic.

' The remaining sodium plumbate slurry, containing a total of approximately 37.5 pounds of free and combined caustic, was then treated with gallons of water, or approximately half the amount of water employed for granulation according to the procedure of Example I. The resulting strongcaustic solution was evaporated to 4 reclaim additional caustic soda solution for reuse, and the hydrated sodium plumbate was hydrolyzed to lead dioxide and washed and dried in the same way as described in Example I. Substantially pure lead dioxide was again produced in good yield.

The process of the present invention is an improved method for the preparation of lead dioxide. It is simple and easily carried out, gives good yields of pure product, and does not result in the preparation of excessive amounts of waste products. The process may be carried out in standard, inexpensive equipment.

While this invention has been described with reference to certain specific embodiments and by way of certain examples, no undue limitations are to be deduced'therefrom, and the invention is not to be limited exceptas set forth in the following claims.

I claim:

1. In a process for the production of lead dioxidewhich comprises the steps of reacting molten lead with caustic alkali and oxidizing with an oxidizing agent such as air or an alkali nitrate to produce a fused mass of alkali plumbate dissolved in caustic alkali, the steps of granulating said alkali plumbate in the presence of from 1.0 to 1.3 parts by weight of water for each part by weight of said fused mass to produce a slurry of solid alkali plumbate in concentrated aqueous caustic alkali, separating said solid alkali plumbate from said concentrated aqueous caustic alkali. hydrolyzing said solid alkali'plumbate with additional 'water to produce lead dioxide and regenerate additional caustic alkali, and separating said regenerated caustic alkali from said lead dioxide.

2. A process according to claim 1, wherein said reaction between molten lead and caustic alkali is carried out at a temperature of at least 327 C.

3. A process for the production of lead dioxide which comprises the steps of reacting molten lead and caustic soda in the presence of sodium nitrate to produce a fused mass of sodium plumbate and sodium nitrite dissolvedin excess caustic soda, granulating said sodium'plumbatedn the presence of from 1.0 to 1.3 parts by weight'ofwater for each part by weight of said fused mass to produce-a slurry of solid sodium plumbate in a concentrated aqueous solution of sodium hydroxide and sodium nitrite, separating said solid sodium plumbate from said aqueous sodiiun hydroxide and sodium nitrite, hydrolyzing said solid sodium plumbate with additional water to produce lead dioxide and regenerate additional sodium hydroxide, and separating said regenerated sodium hydroxide from said lead dioxide.

4. A process for production of lead dioxide, which comprises the steps of reacting molten lead with molten caustic soda and sodium nitrate in amount from 0.8 to 1.0 part by weight of NaNO;, for each part of Pb, at a temperature at least 327 C., to produce sodium plumbate and sodium nitrite dissolved in excess caustic soda, granulating said sodium plumbate in the presence offrom 1.0 to 1.3 parts by weight of water for each part by weight of said fused mass to produce a slurry of solid sodium plumbate in concentrated aqueous caustic soda and sodium nitrite, separating said sodium plumbate from said sodium nitrite and excess caustic soda, separating said sodium nitrite from said excess caustic soda, hydrolyzing said sodium plumbate with water to produce lead dioxide and regenerated caustic soda, and separating said lead dioxide from said regenerated caustic soda.

References Cited in the file of this patent UNITED STATES PATENTS 777,388 McDowell Dec. 13, 1904 2,139,002 Cole Dec. 6, 1938 FOREIGN PATENTS 11,962 Great Britain Aug. 27, 1890 

3. A PROCESS FOR THE PRODUCTION OF LEAD DIOXIDE WHICH COMPRISES THE STEPS OF REACTING MOLTEN LEAD AND CAUSTIC SODA IN THE PRESENCE OF SODIUM NITRATE TO PRODUCE A FUSED MASS OF SODIUM PLUMBATE AND SODIUM NITRITE DISSOLVED IN EXCESS CAUSTIC SODA, GRANULATING SAID SODIUM PLUMBATE IN THE PRESENCE OF FROM 1.0 TO 1.3 PARTS BY WEIGHT OF WATER FOR EACH PART BY WEIGHT OF SAID FUSED MASS TO PRODUCE A SLURRY OF SOLID SODIUM PLUMBATE IN A CONCENTRATED AQUEOUS SOLUTION OF SODIUM HYDROXIDE AND SODIUM NITRITE, SEPARATING SAID SOLID SODIUM PLUMBATE FROM SAID AQUEOUS SODIUM HYDROXIDE AND SODIUM NITRITE, HYDROLYZING SAIS SOLID SODIUM PLUMBATE WITH ADDITIONAL WATER TO PRODUCE LEAD DIOXIDE AND REGENERATE ADDITONAL SODIUM HYDROXIDE, AND SEPARATING SAID REGENERATED SODIUM HYDROXIDE FROM SAID LEAD DIOXIDE. 